The process of food preservation has always been of interest, with freeze drying being of particular food industry focus since the late 1940's. In the formative years of freeze drying apparatus and processing development, there was an emphasis in achieving a useful (preserved) end product. The basic stages of freezing a liquid material and then heating the frozen material under vacuum to effect moisture removal are preferable to earlier attempts at food preservation which often involved dehydration by hot air convection methods.
Dried product quality is of prime importance in terms of nutritional value and appearance to the consumer. It is undesirable to damage the product during the processing and removal of liquids from substances. Hot air convection drying often results in product shrinkage and other detrimental effects.
Batch unit operations have been used in the freeze drying industry although batch processing has a number of disadvantages which include; longer product processing durations, longer residence times at each stage of processing (often as manual labour is required to transfer partially processed substances from one stage to the next), poorly optimised processing equipment often with excess capacity, increased set-up times, reduced control of the process due to increased likelihood of human error (due to lack of automation), and low throughput. Batch systems are typically used for small production runs or where a need for process flexibility is required.
In conventional (batch freeze) drying processes at least one liquid feed is poured into shallow tray(s) (product thickness typically varies between 10 and 20 mm), which are then placed on shelves in the freeze dryer. The door to the batch freezer is closed and the product is frozen. After the product has frozen, the trays are heated and the ice is slowly sublimed. The sublimed vapour is condensed on refrigerated coils. Once it is assumed that the product is dry, the product is removed manually. The product exits a conventional freeze-dryer as a brittle cake, and usually requires a separate granulation stage before it can be further processed.
This method of drying a moisture laden substance by freezing it then subliming off as much excess moisture as possible to produce a dried product is primarily used within industries where substances need to be dried, but are unable to withstand even moderate temperatures, for example some foodstuffs or pharmaceuticals can be damaged or affected by heat.
It is not unusual for this combined freezing and drying process to take 48 hours or more. This is undesirable if heat-sensitive materials are being dried. Also, if the vacuum is lost in a conventional batch freeze-dryer, melting and glass formation may occur, and it is possible that the entire load of product may be lost, that is the product may not be a useable quality product or able to be sold. In addition, the loading and unloading process is susceptible to product contamination due to exposure, and wastage from spillage of the shallow trays.
Therefore, a system which enables rapid freeze drying of a moisture laden substance and which produces a suitably formed end product, such as a powder, is desirable. As yet no truly continuous or semi-continuous freeze drying processes have been effectively developed by the drying industry.
An apparatus able to minimise potential contamination, reduce liquid feed spillage and which may increase the liquid throughput would be advantageous to the drying industry. If a liquid feed is able to be freeze dried which does also not require a necessary pre-treatment liquid feed cooling stages or subsequent dried product granulation stages, then significant problems with conventional batch freeze drying may be overcome.
In more recent times, the stages of freezing and drying have been further developed to include various steps such as reducing the physical size of substances to be treated, as well as graded temperature control during the freezing and drying stages. However, In practice highly controlled drying has often been difficult to implement effectively as the substances being dried are often in static tray type arrangements and therefore some substance is likely to be affected or heated more so than other parts of the substance, therefore resulting in over-dried or overheated and consequently damaged product. It would therefore be advantageous to utilise a drying stage with the provision of improved heat transfer conditions to substances, such a stage being preferably coupled to a freeze dry process.
It is therefore an object of the present invention to provide a dryer apparatus and/or a drying process or method which goes at least some way towards addressing the foregoing problems or to at least to provide the industry with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made, that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.